mlxsw: Use one enum for all registers that contain tunnel_port field

Currently SFN, TNUMT and TNPC registers use separate enums for
tunnel_port.

Create one enum with a neutral name and use it.
Remove the enums that are not currently required.

The next patches add two more registers that contain tunnel_port field,
the new enum can be used for them also.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>